DESCRIPTION (Adapted from Applicant's Description): Although the nervous system probably exerts some level of control over all movement-related variables involved in a task, certain variables are likely to be assigned greater priority than others. A knowledge of the most important control variables for a given task would enhance our ability to evaluate functional performance clinically. Emphasizing such variables when teaching a functional task to patients could lead to more efficient learning and more successful task performance. The project's first aim, therefore, is to develop and evaluate a new method for identifying variables that constitute the nervous system's primary focus of movement control. The approach is motivated by the hypothesis that stability is a signature feature of control. That is, the nervous system keeps variables that are most important for successful task performance more stable than other variables. The sit-to-stand (STS) task is one of the most common activities of daily living. Many patients with neurological deficits (e.g., stroke or cerebral palsy) as well as many elderly individuals have difficulty performing this task. Because knowledge of the motor control of this task is limited, the investigators will use the proposed model to obtain preliminary data on the nervous system's control of this task in healthy subjects as a basis for future experiments on patients and elderly individuals. Thus, the second aim of the project is to identify the primary control variables for the task of standing up from sitting, using the proposed model. The mathematical model will be extended from its current kinematic form to also treat kinetics. Hypotheses about control of the center of mass (CM) under four task conditions will be used to evaluate the kinematic and kinetic versions of the model. The four task conditions are (1) standing up under normal support conditions; standing onto a narrow base of support (2) with and (3) without practice; and (4) standing onto a narrow base of support and immediately grasping a rigid support. Specific hypotheses about what are the primary control variables for the STS task will be tested using stability estimates obtained from the mathematical model for each hypothesized control variable. Kinematic and kinetic data will be collected from 16 healthy adults. The primary dependent variables will be measures of the contribution of joint trajectory variance to the (1) variability and (2) stability of the center of mass, head and hand trajectories, all hypothesized control variables.